Average seat height is 18″.. Another words, from the floor to the top of your cushion should be approximately 18″. So if from the floor to the top of your seat deck, is say 14″, by example, you can add another 4″ in seating height. Keep in mind all foam should be wrapped in batting. So allow for the thickness of batting as well as foam.

This is not a hard fast rule. A persons height may make a difference, But to answer your question about the arm, there are no rules. I suppose you wouldn’t want the cushion to occupy more than half the arm. But again even there, it might be okay on certain types of chairs to exceed more than half the arm. The more important measurement is an average height of 18′ total.